JP2011190506A - Fire-resistant steel material superior in high-temperature strength of base metal and high-temperature ductility of weld heat-affected zone, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fire-resistant steel material which shows high yield strength in each base metal part at 600&deg;C, also has high ductility in a weld heat-affected zone, and is superior in the toughness of the base metal part and the weld heat-affected zone. <P>SOLUTION: The fire-resistant steel material includes 0.005-0.050% C, 0.01-0.50% Si, 0.50-2.00% Mn, 0.50-2.00% Cr, 0.001-0.030% Ti, 0.005-0.10% Al, 0.001-0.006% N, less than 0.01% Mo, less than 0.03% V, 0.0003% or less B, less than 0.02% P, less than 0.01% S, less than 0.01% O and the balance Fe with unavoidable impurities; and has a steel material structure in which one or more of a bainite structure, a martensite structure or a ferrite structure formed of grains with circle equivalent grain sizes of 20 &mu;m or less when measured with a EBSP method occupy 80% or more by an area rate, and the balance is a ferrite or MA structure with unavoidable phases. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a refractory steel material excellent in high-temperature strength of a base material and high-temperature ductility of a weld heat-affected zone, and a method for producing the same.

In general, the strength of steel materials decreases when exposed to high temperatures due to fire or the like. In steel structures such as buildings, when exposed to fire, the steel structure is required to have the performance to secure the escape time of the resident personnel by exhibiting the strength required for a certain period of time and preventing collapse. Therefore, conventionally, a method of covering a steel material with a fireproof coating has been used for the purpose of suppressing the temperature rise of the steel material at the time of a fire.
On the other hand, in recent years, steel materials with characteristics that do not easily decrease strength even at high temperatures, so-called fire resistance performance, have been developed. From the viewpoint of environmental problems and aesthetics, steel structures can be constructed using fire resistance steel without using fire resistance coating. An example of composition has appeared.
The fire resistance required here is determined by the Ministry of Land, Infrastructure, Transport and Tourism based on the “New Fire Resistance Design Act”, and the characteristics are described in the Ministry of Land, Infrastructure, Transport and Tourism Notification No. 333.

In recent years, research and development has been actively conducted on steel materials having fire resistance, and for example, inventions as shown in Patent Documents 1 to 22 are disclosed.
In particular, many component-based inventions that actively use Mo as an alloy element have been disclosed. Patent Documents 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14 , 15, 16 and 22 define a component system premised on the addition of Mo.
These component systems premised on the addition of Mo are intended to promote precipitation of Mo in steel by heating during a fire and to increase the high-temperature strength by precipitation strengthening, and for the purpose of ensuring the fire resistance performance of steel. This method has been adopted by researchers.
Moreover, when manufacturing steel materials using accelerated cooling, hardenability improves by addition of Mo and there exists an effect which raises the room temperature tensile strength of a base material.

  On the other hand, in recent years, the supply and demand of various metal elements has been severe, and in particular, the price of Mo has risen remarkably, and there is also a disclosure of a technique for designing an alloy that does not necessarily depend on Mo. Patent Documents 12 and 18 disclose inventions that utilize Cu precipitation as an alloy element.

  In addition, in order to ensure the high temperature strength of the steel material, in addition to the above-described method using precipitation of alloy elements, there is also a method in which the steel material structure is a fine ferrite structure of crystal grains or a bainite structure having a high dislocation density. Patent Document 17 discloses an invention premised on the addition of B in order to obtain a bainite structure. Patent Documents 3, 4, 7, 9, 10, 12, 14, 16, 19, and 20 disclose a technique that uses accelerated cooling to obtain a bainite structure.

JP-A-8-134484 Japanese Patent Laid-Open No. 10-046284 Japanese Patent Laid-Open No. 10-068018 Japanese Patent Laid-Open No. 10-096024 JP-A-10-121194 Japanese Patent Laid-Open No. 10-204529 Japanese Patent Laid-Open No. 11-050198 JP-A-11-131175 JP 2000-192142 A JP 2000-248317 A JP 2001-294984 A Japanese Patent Laid-Open No. 2002-11502 JP 2004-084068 A JP 2005-272949 A JP 2006-249467 A JP 2007-051321 A JP 2007-191746 A JP 2007-191747 A JP 2007-2111278 A JP 2007-277679 A JP 2007-277680 A JP 2007-291383 A

In the development of conventional refractory steel materials, it has been insufficient to pursue the point of ensuring the deformability when a welded joint using refractory steel materials is heated to a high temperature by a fire.
When the welded structure is exposed to fire, even if the high-temperature yield strength and high-temperature tensile strength of the base metal and the weld heat-affected zone (hereinafter referred to as HAZ) are sufficiently secured, There are cases in which the ductility (deformability) at is significantly reduced.

If a welded structure is constructed using steel material with low ductility at high temperature of HAZ, when HAZ is heated by a fire, the deformability of HAZ is insufficient, and as a result, the welded structure can break from HAZ and collapse. As a result of the present inventors' research, it has been clarified that there is a characteristic.
Therefore, it is difficult to design a welded structure using a refractory steel material unless a steel material in which the ductility of HAZ at a high temperature is ensured is developed.

  The phenomenon that ductility decreases when HAZ is exposed to high temperatures (hereinafter referred to as HAZ reheat embrittlement) is caused by the fracture of the steel material along the old γ grain boundary in the bainitic structure unique to HAZ. However, the results of studies by the present inventors have become clear.

The HAZ reheat embrittlement phenomenon has been known in the field of some high-strength steels and heat-resistant steels, and research has been conducted to determine the cause of precipitation of carbides and nitrides, or segregation of impurities at the former γ grain boundaries. Although there are many reports, there is no interpretation that can explain the phenomenon uniformly with various steel materials.
Therefore, the present inventors first examined in detail the effects of various alloy elements on the reheat embrittlement of HAZ through experiments and analyzes in the development of this refractory steel.

Patent Documents 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, and 22 define a component system premised on the addition of Mo. This is because the precipitation of Mo carbides when exposed to high temperatures is used as a means for increasing the yield strength at high temperatures. It was confirmed that a large amount of Mo carbide was precipitated at the old γ grain boundary, and reheat embrittlement of HAZ became remarkable.
In addition, with the recent increase in the price of Mo, the alloy costs are extremely high in the component systems described in these documents, and at the same time, there is a problem that it does not meet the market price of refractory steel materials.

  Patent Documents 15, 16, 17 and 22 employ a component system premised on the addition of B, but in these component systems, a large amount of B nitride is generated at the former γ grain boundary of the HAZ bainite structure, The present inventors have confirmed through experiments that the reheat embrittlement of HAZ becomes significant.

  In Patent Documents 7, 11 and 13, since the C addition amount is specified to be 0.05% or more, in these component systems, a large amount of carbides of various alloy elements are generated at the former γ grain boundaries of the HAZ bainite structure. The present inventors have confirmed by experiments that the reheat embrittlement of HAZ becomes remarkable.

  In Patent Documents 12 and 18, the Cu addition amount is specified to be 0.6% or more and 0.5% or more, respectively, and in these component systems, Cu precipitates are formed at the old γ grain boundaries of the HAZ bainite structure. The present inventors have confirmed through experiments that the reheating embrittlement of the HAZ becomes remarkable due to the occurrence of the formation of fine austenite structure.

In Patent Documents 19, 20, and 21, the Nb addition amount is defined as a high level of 0.05% or more and 0.50% or less. In these component systems, C that promotes reheat embrittlement of HAZ, The present inventors have confirmed through experiments that the combination of elements such as Mn, V, Mo, or Cu cannot stably suppress reheat embrittlement of HAZ.
As described above, in the conventional refractory steel development, the pursuit of HAZ reheat embrittlement was insufficient.

  Furthermore, the refractory steel is also required to exhibit a high yield strength when heated to a high temperature by a fire. In general, it is considered that the strength of a steel material at a high temperature is expressed by dislocation strengthening due to dislocations existing in the steel material, strengthening by crystal grain refinement, and precipitates that hinder dislocation motion. Therefore, in order to ensure high high-temperature yield strength, it is effective to have a sufficient amount of dislocations, or to include many precipitates and grain boundaries that hinder the movement of dislocations. is there.

  Patent Documents 13, 17, 21, and 22 disclose a manufacturing method in which cooling is performed without using accelerated cooling after hot rolling. In such a manufacturing method, in order to ensure the amount of dislocations and precipitates in the steel material necessary to ensure high-temperature yield strength, a large amount of alloy elements that improve hardenability and alloy elements that cause precipitation strengthening. Although the use of alloying elements such as Mo, Nb, or B, which are specified in these patent documents, is effective for improving the high-temperature yield strength, the HAZ is reheated. As described above, the result of promoting embrittlement is brought about.

  In addition, the market price of steel materials having fire resistance is not sufficiently high compared to general-purpose steel not having fire resistance. Therefore, when manufacturing a steel material having fire resistance, it is desirable to suppress the addition of various alloys as much as possible in order to reduce the alloy cost.

As a method for ensuring high-temperature yield strength without depending on the addition of such alloy elements, a technique for obtaining a bainite structure having a high dislocation density by using accelerated cooling in the steel material manufacturing process is disclosed in Patent Documents 3, 4, and 7. 9, 10, 12, 14, 16, 19 and 20.
However, the method of obtaining a bainite structure using accelerated cooling has a problem that not only the high-temperature yield strength but also the room temperature tensile strength is remarkably increased depending on the design of the alloy component system.

In many cases, refractory steel materials require mechanical properties characterized by the fact that the tensile strength at room temperature falls within a certain range of lower and upper limits, and that the yield strength at high temperatures is above a certain value. Is done.
Therefore, when pursuing high-temperature yield strength, if you try to obtain a steel structure with a high dislocation density by using accelerated cooling, until the room temperature tensile strength deviates from or exceeds the upper limit required for the steel. Becomes close to the upper limit value, and there is a problem that the tolerance for material fluctuation in manufacturing the actual machine is lost.

  Patent Documents 3, 4, 7, 9, 10, 14 and 16 disclose a manufacturing method using accelerated cooling in a component system premised on addition of Mo, and Patent Document 16 discloses a component system premised on addition of B. However, in these component systems, the hardenability of Mo or B appears prominently and the room temperature tensile strength is increased, so that the tolerance for the material variation in manufacturing the actual machine described above is lost.

  In Patent Document 12, in a component system in which Cu is added by 0.6% or more and Ni is added by more than half of the Cu amount, and in Patent Documents 19 and 20, in a component system in which Nb is added by 0.05% or more, Although a manufacturing method using accelerated cooling is disclosed, the hardenability is increased more than necessary even in these component systems, and the tolerance for the material fluctuation in manufacturing the actual machine described above is lost.

In recent years, buildings have become larger and higher-rise for the purpose of effective use of land, and the steel materials used there have also tended to increase in size. Heat tends to be high.
Therefore, in order to obtain sufficient earthquake resistance even when the welding heat input is high, the toughness of the welded portion must be sufficiently high. The present invention addresses such a problem at the same time, and presupposes that the alloy element addition amount is optimized to obtain the toughness of the welded HAZ when heat input of 5 kJ / mm or more is applied.
Furthermore, since it is also important to secure a sufficient base material toughness as a steel material used for construction or the like, the present invention also presupposes the acquisition of the base material toughness.

  The present invention has been made in view of the above circumstances, and relates to refractory steel materials used for steel structures such as high-rise buildings and parking lots, and the yield of the base material portion is high at 600 ° C. when exposed to fire. Refractory steel material having strength and at the same time having high ductility at 600 ° C. when the weld heat affected zone of the steel material is exposed to a fire, and further excellent toughness of the base metal portion and the weld heat affected zone, and a method for producing the same The purpose is to provide.

The most important problem of the present invention is that a steel material having a room temperature tensile strength of 450 to 650 MPa has a yield strength at 600 ° C. of 217 MPa or more and sufficient deformation when the HAZ is reheated to an assumed temperature of 600 ° C. at the time of fire. It is to provide a refractory steel material having the characteristics of having ability.
As already mentioned, in order to obtain high-temperature yield strength, the use of precipitates such as carbides and nitrides, the use of a fine ferrite structure of crystal grains, or the use of a bainite structure with a high dislocation density. Although it is effective to use, the addition of various precipitated elements tends to promote reheat embrittlement of HAZ, and the indiscriminate addition of hardenability improving elements keeps the room temperature tensile strength within the required level range. There is a problem of losing.

The inventors have studied in detail the effects of various alloying elements on high-temperature yield strength, room-temperature tensile strength, and HAZ reheat embrittlement through experiments and analyzes in the production of refractory steel by a process using accelerated cooling. We have found metallurgical knowledge to keep all of these within the target range, and have developed appropriate component systems and processes.
The target of the present invention here is that the yield strength at 600 ° C. is 217 MPa or higher, the room temperature tensile strength is 450 MPa or higher and 650 MPa or lower, and the weld heat-affected zone is heated to 600 ° C. or higher. Is 25% or more.

  The present inventors first obtained an alloy component for securing high-temperature yield strength in a state where room temperature tensile strength is suppressed in obtaining a fine ferrite structure, bainite structure, or martensite structure of crystal grains using accelerated cooling. The system and manufacturing process were examined.

In order to lower the room temperature tensile strength in the fine ferrite structure, bainite structure, or martensite structure of the crystal grains, it has been found that it is most important to reduce the C content first. This has been found to be possible because it is possible to reduce the amount of solid solution C in the steel material and the amount of cementite finely precipitated.
In addition, the positive addition of Cr has the effect of reducing the hardenability by combining the solid solution C and Cr in the steel material, and contributes to the increase in high-temperature yield strength by precipitation of Cr carbide. Newly discovered.

In addition, the present inventors clarified the influence of various alloy elements on the reheat embrittlement of HAZ through experiments. As a result, it has been found that most of the elements forming carbide or nitride, such as Mo, B, Nb, and V, precipitate at the former γ grain boundaries of HAZ and promote reheat embrittlement.
In addition, elements such as Mn, W, Cu, etc. that themselves do not form carbides or nitrides and contribute to improvement of hardenability also generate fine austenite structures at the former γ grain boundaries of HAZ and promote grain boundary sliding. It was found that this promotes reheat embrittlement of HAZ.

On the other hand, it is also clarified that Cr and Ti have an effect of improving the reheat embrittlement of HAZ.
Regarding Cr, it has the effect of suppressing carbide precipitation of other elements by forming Cr carbide, and the size of Cr carbide stays in the cluster of several to several tens of nanometers. It turned out that it was difficult to contribute.
Regarding Ti, since Ti carbides and nitrides are precipitated regardless of grain boundaries and within grains, the effect of reducing the total amount of carbides and nitrides of other elements precipitated at the former γ grain boundaries of HAZ appears. Turned out to be.

In the present invention, based on the above knowledge, an alloy component system for suppressing reheat embrittlement of HAZ has been examined based on experiments and analysis. The feature of the present invention is that Cr and Ti are actively used in a low C component system. Impurity levels of Mo, Nb, and B that promote excessive increase in room temperature tensile strength and HAZ reheat embrittlement. The content is limited.
When the component system of the present invention has a reduction ratio at 1000 ° C. or lower and 800 ° C. or higher of 50% or higher, the hot rolling is finished at 800 ° C. or higher, and immediately thereafter, a cold speed of 2 ° C./second or higher to a temperature range of 600 ° C. Accelerated cooling at room temperature can provide the properties that the room temperature tensile strength is 450 MPa or more, 650 MPa or less, the base material yield strength at 600 ° C is 217 MPa or more, and the fracture drawing value in the 600 ° C tensile test of the HAZ part is 30% or more. I can do it.

In addition, by tempering at a temperature of 400 ° C. or higher and 650 ° C. or lower after accelerated cooling, it is possible to lower only the room temperature tensile strength without lowering the high-temperature yield strength and further improve the toughness of the base material. .
In addition, it is also possible to ensure the toughness of the HAZ at the time of high heat input welding with a heat input of 5 kJ / mm or more by appropriately selecting various alloy elements within the alloy component range of the present invention.

The gist of the present invention based on the above findings is as follows.
(1) The chemical composition is mass%,
C: 0.005% or more, 0.050% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.50% or more, 2.00% or less,
Cr: 0.50% or more, 2.00% or less,
Ti: 0.001% or more, 0.030% or less,
Al: 0.005% or more, 0.10% or less,
N: 0.001% or more, 0.006% or less,
Containing
Mo: less than 0.01%,
V: less than 0.03%,
B: 0.0003% or less,
P: less than 0.02%,
S: less than 0.01%,
O: Steel material consisting of the remaining Fe and inevitable impurities limited to less than 0.01%, and 80% or more by area ratio of the steel material structure is bainite structure, martensite structure, or EBSP (Electron Back Scattering Pattern) ) A ferrite or MA (martensite-austenite mixture) structure with an equivalent circle diameter of more than 20 μm and an inevitable structure with an equivalent circle diameter measured by the method of 20 μm or less. The tensile strength at room temperature is 450 MPa or more and 650 MPa or less, the 600 ° C. yield stress of the base material of this steel material is 217 MPa or more, and the fracture drawing value of the 600 ° C. high temperature tensile of the weld heat affected zone of this steel material is The high-temperature strength of the base metal and the high heat-affected zone of the base metal, characterized by being 25% or more Refractory steel with excellent hot-rollability.

(2) In addition to (1),
Nb: 0.001% or more, 0.10% or less,
Ni: 0.01% or more, 1.00% or less,
Cu: 0.01% or more, 0.10% or less,
W: 0.01% or more, 0.50% or less,
The refractory steel material excellent in the high temperature strength of the base material and the high temperature ductility of the weld heat affected zone according to (1), characterized by containing one or more of the above.

(3) In addition to (1) or (2), in mass%,
Zr: 0.001% or more, 0.050% or less,
Mg: 0.0005% or more, 0.0050% or less,
Ca: 0.0005% or more, 0.0050% or less,
Y: 0.001% or more, 0.050% or less,
La: 0.001% or more, 0.050% or less,
Ce: 0.001% or more, 0.050% or less,
The refractory steel material excellent in the high temperature strength of the base material and the high temperature ductility of the weld heat affected zone according to (1) or (2), characterized by containing one or more of the above.

(4) After heating the steel slab having the steel component described in (1) to (3) to 1000 ° C. or higher and 1300 ° C. or lower, and performing hot working or hot rolling, 800 ° C. or higher and 1000 ° C. or lower. At 50 ° C. or higher, hot rolling or processing is finished at 800 ° C. or higher, and then accelerated cooling at a cooling rate of 2 ° C./second or higher from a temperature range of Ar 3 or higher to a temperature range of 600 ° C. or lower. A method for producing a refractory steel material excellent in high-temperature strength of a base material and high-temperature ductility of a weld heat-affected zone.

(5) After applying the production method according to (4), the steel material is subjected to a tempering heat treatment in a temperature range of 400 ° C. or more and less than 650 ° C. for 5 minutes or more and 360 minutes or less, and the high temperature of the base material A method for producing a refractory steel material excellent in strength and high temperature ductility of a weld heat affected zone.

  According to the present invention, it is possible to stably obtain a steel material having a fire resistance performance and a room temperature tensile strength of 450 to 650 MPa in an economical component system having a small number of alloy elements. Moreover, since the steel material of this invention can fully ensure a deformability, even when the heat affected zone of a welded joint is exposed to a fire, the contribution to the safety ensuring of the building using a refractory steel material is very large.

The reason for limiting the chemical component composition in the present invention will be described below.
C is an element effective for improving hardenability, and is added in an amount of 0.005% or more. However, if added over 0.05%, a martensite-austenite mixed structure (in HAZ during large heat input welding) (Hereinafter referred to as MA structure), which may significantly deteriorate the toughness of HAZ, increase the amount of carbides precipitated at the grain boundaries of HAZ during a fire, and cause reheating embrittlement of HAZ, and In order to cause an excessive increase in the tensile strength at room temperature, the addition range is set to 0.005% or more and 0.05% or less.

  Si is added in an amount of 0.01% as a deoxidizing element, but if added in excess of 0.50%, the formation of MA structure may be promoted in the HAZ during high heat input welding and the toughness may be reduced. Therefore, the addition range is set to 0.01% or more and 0.50% or less.

  Mn is effective in improving hardenability, and it is necessary to add 0.50% or more in order to ensure the room temperature tensile strength of 450 MPa or more, which is the object of the present invention, but it exceeds 2.0%. Then, since the reheat embrittlement of HAZ is promoted and the room temperature tensile strength is excessively increased, the upper limit of the addition amount is limited to 2.0%.

  Cr has the effect of improving the high-temperature yield strength without increasing the room-temperature tensile strength, and has the effect of suppressing reheat embrittlement of HAZ, so 0.5% or more is added. However, if added over 2.0%, there is no particular adverse effect on the steel material properties, but the steelmaking problem, in particular the extension of the impurity removal time, lowers the molten steel temperature during refining, and deteriorates the castability. The upper limit of addition is limited to 2.0%. In addition, in this invention, it is preferable to increase Cr amount as a countermeasure, so that many elements which promote reheat embrittlement of HAZ, such as C, Mn, or Cu, are added.

  Ti is added in an amount of 0.001% or more in order to remarkably suppress reheat embrittlement of HAZ. However, if added over 0.030%, the toughness of the base material is significantly reduced, so the upper limit is limited to 0.030%.

  Al is added in an amount of 0.005% as a deoxidizing element, but if added over 0.10%, the formation of MA structure may be promoted in the HAZ during high heat input welding and the toughness may be reduced. Therefore, the addition range is made 0.005% or more and 0.10% or less.

  N forms 0.001% or more in order to form nitrides with various alloy elements and contribute to the improvement of high-temperature yield strength. However, if a large amount is added, the nitride that precipitates at the grain boundaries of the HAZ during a fire becomes coarse and reheat embrittlement becomes prominent, so the upper limit is limited to 0.006%.

  Mo contributes to an increase in room temperature tensile strength and high temperature yield strength by improving hardenability and carbide precipitation. However, Mo is not preferable in the present invention because Mo precipitates coarsely at the grain boundaries of HAZ as a carbide or Laves phase during fire, and reheat embrittlement becomes remarkably remarkable. Therefore, even if a small amount of contamination due to industrial production is unavoidable, aggressive addition should be refrained, and the content is made less than 0.01% due to the industrial production margin.

  V may increase the Ar3 point in the range of the component system of the present invention. In that case, when the water cooling start temperature is lower than the Ar3 point, coarse ferrite grains are generated and the high temperature yield strength is lowered. The existence of is confirmed. Therefore, in the present invention, V is not actively added from the viewpoint of production stability, and the content is limited to less than 0.03% as an inevitable mixing level due to industrial production.

  B contributes to an increase in room temperature tensile strength and high-temperature yield strength by improving hardenability and nitride precipitation. However, the addition of B is not preferable in the present invention because the nitride of B tends to coarsely precipitate at the grain boundaries of the HAZ during a fire and reheat embrittlement becomes remarkably remarkable. Therefore, even if a small amount of contamination due to industrial production is unavoidable, aggressive addition should be refrained, and the upper limit of the addition amount is set to 0.0003% from the margin of industrial production.

P, as an impurity, remarkably lowers the toughness of the base material and also makes the HAZ reheat embrittlement significant in the event of a fire, so limits the amount added to less than 0.020%.
S, as an impurity, remarkably lowers the toughness of the base material and also makes reheating embrittlement of the HAZ during a fire remarkable, so the addition amount is limited to less than 0.010%.
O as an impurity remarkably lowers the toughness of the base metal and also makes reheating embrittlement of the HAZ during a fire remarkable, so the addition amount is limited to less than 0.010%.

  Nb increases the hardenability of the steel material and contributes to the improvement of the room temperature tensile strength, and also precipitates as a carbide or nitride and contributes to the improvement of the high temperature yield strength. In order to obtain this effect, addition of 0.001% or more is necessary. However, excessive addition makes the reheating embrittlement of HAZ noticeable. Therefore, when adding, the range is limited to 0.001% or more and 0.10% or less.

  Ni is effective for increasing the room temperature tensile strength and the high temperature yield strength by improving the hardenability. In order to obtain this effect, addition of 0.01% or more is necessary. However, addition of 1.00% or more may significantly reduce the toughness of HAZ during high heat input welding. In such a case, the range is limited to 0.01% or more and 1.00% or less.

  Cu is effective in increasing the room temperature tensile strength by improving hardenability and increasing the high temperature yield strength by precipitation. In order to obtain this effect, addition of 0.01% or more is necessary. However, if the addition exceeds 0.10%, reheating embrittlement of HAZ becomes remarkable. Therefore, when added, the range is limited to 0.01% or more and 0.10% or less.

  W is effective in improving hardenability and increasing room temperature tensile strength and high temperature yield strength by solid solution strengthening. To obtain this effect, 0.01% or more must be added. Further, W has a lower diffusion rate in ferrite or bainite than Mo, and precipitation of HAZ at the former γ grain boundary is slower than Mo, so it is difficult to promote reheat embrittlement of HAZ. However, if 0.50% or less is added, the reheat embrittlement of the HAZ becomes significant. Therefore, when added, the range is limited to 0.01% or more and 0.50% or less.

  Zr precipitates as carbides and nitrides and contributes to an increase in high-temperature yield strength. In order to obtain this effect, addition of 0.001% or more is necessary, but if added over 0.050%, the carbides precipitated at the grain boundaries are coarsened, and HAZ reheat embrittlement becomes prominent. When adding, it is made 0.001% or more and 0.050% or less.

  Mg has the effect of controlling the form of sulfide in the steel material and reducing the reduction in the base metal toughness due to sulfide. In order to obtain this effect, addition of 0.0005% or more is necessary. However, since the effect is saturated when 0.0050% or more is added, the range of addition is 0.005% or more, and 0.005% or more. Limit to 0050%.

  Ca is effective as a deoxidizing element, and has an effect of controlling the form of sulfide in the steel material and reducing the decrease in the base material toughness. In order to obtain these effects, addition of 0.0005% or more is necessary. However, since the effect is saturated with addition of 0.0050% or more, the range of addition is 0.0005% or more, 0 Limit to within .005%.

  Y has the effect of controlling the form of the sulfide in the steel material and reducing the decrease in the base metal toughness due to the sulfide. In order to obtain this effect, addition of 0.001% or more is necessary, but addition of 0.050% or more causes precipitation as a coarse oxide cluster and decreases toughness. Is limited to 0.001% or more and 0.050% or less.

  La has an effect of controlling the form of sulfide in the steel material and reducing a decrease in base material toughness due to sulfide. In order to obtain this effect, addition of 0.001% or more is necessary, but addition of 0.050% or more causes precipitation as a coarse oxide cluster and decreases toughness. Is limited to 0.001% or more and 0.050% or less.

  Ce has an effect of controlling the form of the sulfide in the steel material and reducing a decrease in the base material toughness due to the sulfide. In order to obtain this effect, addition of 0.001% or more is necessary, but addition of 0.050% or more causes precipitation as a coarse oxide cluster and decreases toughness. Is limited to 0.001% or more and 0.050% or less.

  By limiting the above alloy elements, a steel material having a high yield strength at 600 ° C., preventing reheating embrittlement of the HAZ at the time of fire, and excellent in weld HAZ toughness with a heat input of 5 kJ / mm and base metal toughness is obtained. Things will be possible.

Furthermore, the reasons for limiting the manufacturing method in the present invention will be described.
In the present invention, the room temperature tensile strength is 450 MPa or more and 650 MPa or less, the yield strength at 600 ° C. is 217 MPa or more, reheating embrittlement of HAZ is prevented, and toughness is secured even in HAZ by welding with a heat input of 5 kJ / mm. In addition, we have proposed a component system that is a necessary condition for ensuring the toughness of the base metal, and by applying hot rolling and accelerated cooling that define the temperature and reduction amount to the steel slab having the component, It is possible to produce a steel material that satisfies all of the above characteristics.

  Generally, the high-temperature yield strength is considered to be manifested by dislocation strengthening due to dislocations existing in the steel material and crystal grain boundaries and precipitates that hinder dislocation motion. In particular, when the temperature of the steel material exceeds 550 ° C. and the coalescence of dislocations occurs due to the upward movement of dislocations, the high-temperature yield strength may suddenly decrease.

  For this reason, in order to ensure high high-temperature yield strength, the steel material has a sufficient amount of dislocations at the time before exposure to fire, that is, at room temperature, or it becomes an obstacle to the movement of dislocations. It is effective to include many structures, specifically precipitates and crystal grain boundaries. In addition, about a deposit, you may produce | generate for the first time in the stage exposed to a fire.

  The present inventors made 80% or more of the steel structure a fine ferrite structure of crystal grains or a bainite or martensite structure with a high dislocation density, and in the case of bainite or martensite, it is possible to refine the structure at a high temperature. It has been clarified through experiments and analyzes that it is effective in increasing yield strength and improving toughness. The remainder becomes an inevitable phase such as a ferrite structure with a crystal grain diameter equivalent to a circle of more than 20 μm, an MA (martensite-austenite mixture) structure, or inclusions. In the case of a ferrite structure, although it has excellent toughness, it has been found by examination of the present inventors that there are many cases in which the high-temperature yield strength is remarkably reduced especially when the average of the equivalent circle diameter of ferrite crystal grains measured by EBSP exceeds 20 μm. In order to exhibit stable high-temperature strength, it is necessary to suppress the ferrite structure having a circle equivalent particle size exceeding 20 μm to less than 20% of the steel material structure.

In order to obtain a steel structure having the above characteristics, the present inventors, when hot rolling or hot working a steel slab having the chemical component of the present invention, a temperature range of 1000 ° C. or lower and 800 ° C. or higher. The amount of reduction or machining is increased with 50% or more, and then accelerated cooling is performed at a cooling rate of 2 ° C / second or more from the temperature range of Ar3 or higher to 600 ° C or lower. It was found by experiment and analysis that it is effective to do.
By performing the processing of 50% or more, it becomes possible to stably secure the base material toughness within the scope of the present invention.
Further, in order to perform accelerated cooling from the temperature range of the Ar3 point or higher to the temperature range of 600 ° C or lower after rolling or processing, it is necessary to end the hot rolling or processing at 800 ° C or higher.

  In general, when the amount of reduction in the austenite region is increased, the Ar3 point rises, and bainite transformation or martensitic transformation does not easily occur and ferrite transformation is likely to occur. Further, when the ferrite transformation starts just below the Ar3 point, the ferrite grows quickly and the crystal grain size tends to become coarse. Therefore, after hot rolling or hot working, accelerated cooling needs to be started before the temperature drops to the Ar3 point.

Furthermore, in the present invention, tempering heat treatment can be applied after accelerated cooling. Applying tempering heat treatment further promotes precipitation of alloy elements remaining in a solid solution state without being completely precipitated as it is cooled after hot rolling, and further increases the number of precipitates that suppress the reduction of dislocations during a fire. It is possible to make it.
This tempering can be appropriately selected between 400 ° C. and less than 650 ° C. to determine the temperature, and the effect of the present invention can be enhanced by determining the required material strength and the type of alloy element to be precipitated. .
The same applies to the tempering time. When the change in structure during tempering is governed by the diffusion of the substance, increasing the temperature and increasing the time have the same effect, so that the time varies from 5 minutes to 360 depending on the tempering temperature. Can be determined in minutes.

As described above, in the present invention, it is important to effectively use the precipitation of alloy elements. As a means for stably and reliably obtaining such precipitation of alloy elements, When rolling, it is necessary to heat to 1000 ° C. or higher and 1300 ° C. or lower.
This is achieved by heating to a temperature of 1000 ° C. or higher to completely or as much as possible dissolve solid carbides or nitrides of various alloy elements, for example, TiC, ZrC, Cr23C6, etc. The purpose is to adjust the hardenability after hot rolling and increase the stability in manufacturing the actual machine.

Without this heating, alloy elements such as C, Cr, Ti and Zr are already coarsely precipitated before hot rolling, etc., and the reduction in dislocation density of the steel due to the decrease in hardenability after hot rolling. In addition, the amount of precipitation strengthening is reduced due to the reduction of fine carbides or nitrides precipitated after hot rolling, which greatly affects the stability of the manufactured steel at room temperature tensile strength and high temperature yield strength.
However, if the heating temperature exceeds 1300 ° C., the increase in the oxide scale on the surface of the steel material becomes significant, so the upper limit of the heating temperature is limited to 1300 ° C.
As described above, by combining the limitation of the manufacturing technology in addition to the limitation of the chemical composition, it is possible to provide a refractory steel material excellent in high-temperature yield strength with the best yield and optimized alloy addition amount. Become.

  Some examples of the present invention are shown below. Table 1 shows the chemical composition of the steel material, Table 2 shows the manufacturing plate thickness, heating temperature, 1000-800 ° C. rolling reduction ratio (total rolling reduction rate when the surface temperature of the rolled steel material immediately before rolling is between 800 ° C. and 1000 ° C.) and heat. Hot rolling conditions, cooling rate (average cooling rate from accelerated cooling start to water cooling stop temperature performed immediately after hot rolling finish), water cooling stop temperature, tempering temperature, and room temperature tensile strength (room temperature TS) ), Room temperature yield strength (room temperature YS), 600 ° C. yield strength (600 ° C. YS), fracture drawing value of 600 ° C. tensile test of HAZ, absorbed energy of base metal Charpy test at 0 ° C., and HAZ Charpy test at 0 ° C. Shows the absorbed energy. In Tables 1 to 3, items that do not fall within the scope of the present invention are displayed with an underline.

  The room temperature tensile test was carried out based on JISZ2241, and when the upper yield point appeared on the stress-strain curve, the upper yield point was taken as room temperature yield strength, and when it did not appear, 0.2% proof stress was taken as room temperature yield strength. The target of room temperature tensile strength is 450 MPa or more and 650 MPa or less, and the target of room temperature yield strength is 325 MPa or more.

  The high temperature tensile test was performed at 600 ° C. based on JISG0567, and the measured 0.2% proof stress was defined as 600 ° C. yield strength. The target of 600 ° C. yield strength is 217 MPa or more.

  The 600 ° C. tensile test of HAZ gives a heat history assuming a heat input of 2 kJ / mm to the steel slab, then raised the temperature from room temperature to 600 ° C. over 20 minutes, held at 600 ° C. for 30 minutes, and then strain rate A tensile test was carried out at 0.10% / second, and the drawing value (cross-sectional area reduction rate) of the fractured portion of the test piece was measured and used as an index of HAZ reheat embrittlement. The target of this index is 25% or more.

  The Charpy test of the base material was carried out by taking a 2 mmV impact test piece in accordance with JISZ2202 from the thickness position 1 / 2t of each steel material and performing an impact test method in accordance with JISZ2242. The target of absorbed energy was set to 27J or more in consideration of the earthquake resistance of the building structure.

  The HAZ Charpy test is performed by applying a thermal cycle assuming a heat input of 5 kJ / mm to each steel material, collecting a 2 mm V notch impact test piece in accordance with JISZ2202, and performing an impact test method in accordance with JISZ2242. It was. The target of absorbed energy was 27J considering the earthquake resistance of the building structure.

  Further, the structure fractions of ferrite, bainite and martensite having an equivalent crystal grain diameter of 20 μm or less, measured by an EBSP (Electron Back Scattering Pattern) method, calculated from the result of optical microscope observation at the center position of the thickness of the steel structure are shown. This is shown in FIG. The sum of these values is characterized by 80% or more in the present invention.

  In addition, the optical microscope observation was performed about the visual field of 500 micrometers x 500 micrometers after corroding the sample extract | collected from the plate | board thickness center position of each steel material with a Nital reagent. Similarly, the EBSP observation was performed in a grid pattern at intervals of 0.5 μm for a field of view of 500 μm × 500 μm after electrolytic polishing of a sample collected from the center position of the thickness of each steel material. The equivalent circle crystal grain size of ferrite measured by EBSP is defined as a crystal grain boundary between measurement points having a crystal orientation difference of 15 ° C. or more, and a circle with an equal area is considered for each crystal grain. It refers to the diameter when

From the results in Table 3, it can be seen that those within the range of the examples of the present invention all have room temperature tensile strength, room temperature yield strength, and 600 ° C. yield strength within the target range.
At the same time, it can be seen that the fracture drawing value in the 600 ° C. tensile test of the welded HAZ, which is an important feature of the present invention, is secured at 25% or more, and the high temperature ductility of the HAZ is secured.
Further, in the range of the examples of the present invention, the absorption energy of the Charpy test of the base material and the HAZ is 27 J or more at 0 ° C.
From the results shown in Table 3, the comparative steels that do not fall within the scope of the present invention are: room temperature tensile strength, room temperature yield strength, 600 ° C. yield strength, HAZ 600 ° C. tensile test fracture drawing value, base metal Charpy absorbed energy at 0 ° C. , Or HAZ Charpy absorbed energy at 0 ° C. does not meet the target.

Claims (5)

Chemical composition is mass%,
C: 0.005% or more, 0.050% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.50% or more, 2.00% or less,
Cr: 0.50% or more, 2.00% or less,
Ti: 0.001% or more, 0.030% or less,
Al: 0.005% or more, 0.10% or less,
N: 0.001% or more, 0.006% or less,
Containing
Mo: less than 0.01%,
V: less than 0.03%,
B: 0.0003% or less,
P: less than 0.02%,
S: less than 0.01%,
O: Steel material consisting of the remaining Fe and inevitable impurities limited to less than 0.01%, and 80% or more by area ratio of the steel material structure is bainite structure, martensite structure, or EBSP (Electron Back Scattering Pattern) ) A ferrite or MA (martensite-austenite mixture) structure with an equivalent circle diameter of more than 20 μm and an inevitable structure with an equivalent circle diameter measured by the method of 20 μm or less. The tensile strength at room temperature is 450 MPa or more and 650 MPa or less, the 600 ° C. yield stress of the base material of this steel material is 217 MPa or more, and the fracture drawing value of the 600 ° C. high temperature tensile of the weld heat affected zone of this steel material is The high-temperature strength of the base metal and the high heat-affected zone of the base metal, characterized by being 25% or more Refractory steel with excellent hot-rollability. In addition to claim 1,
Nb: 0.001% or more, 0.10% or less,
Ni: 0.01% or more, 1.00% or less,
Cu: 0.01% or more, 0.10% or less,
W: 0.01% or more, 0.50% or less,
The refractory steel material excellent in the high temperature strength of the base material and the high temperature ductility of the weld heat affected zone according to claim 1, characterized by containing one or more of the above. In addition to claim 1 or 2,
Zr: 0.001% or more, 0.050% or less,
Mg: 0.0005% or more, 0.0050% or less,
Ca: 0.0005% or more, 0.0050% or less,
Y: 0.001% or more, 0.050% or less,
La: 0.001% or more, 0.050% or less,
Ce: 0.001% or more, 0.050% or less,
A refractory steel material excellent in high-temperature strength of a base material and high-temperature ductility of a weld heat-affected zone, characterized by containing one or more of them.   The steel slab having the steel component according to any one of claims 1 to 3 is heated to 1000 ° C or higher and 1300 ° C or lower, and then subjected to hot working or hot rolling. Rolling or processing is performed, hot rolling or processing is finished at 800 ° C or higher, and then accelerated cooling is performed at a cooling rate of 2 ° C / second or higher from a temperature range of Ar3 or higher to a temperature range of 600 ° C or lower. The manufacturing method of the refractory steel materials excellent in the high temperature strength of a preform | base_material, and the high temperature ductility of a welding heat affected zone.   After applying the manufacturing method according to claim 4, the steel material is subjected to a tempering heat treatment in a temperature range of 400 ° C or higher and lower than 650 ° C for 5 minutes or more and 360 minutes or less. A method for producing a refractory steel material excellent in high temperature ductility of a heat affected zone.